Implement showing a long datetime as a tooltip when hovering over the timestamp for a message. The timestamp is also moved to just after the username of the poster this means it will not be under the hover buttons unless the message is very short.
Also some cleanup of the alignment of items in the bubble. The reply text is now indented the same amount as the username and message and the padding isn't removed from the username and message when avatars are disabled.
Implements: network/neochat#223
